<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8"/>
    <title>Title</title>
</head>
<body>
    <script>
        for(var i=1;i<=6;i++){
            if(i==3){
                document.write('<h3>'+'三级标题'+'</h3>');
            }
            else if(i!=3){
                document.write('<h'+i+'>'+i+'级标题'+'</h'+i+'>');
            }
        }
    </script>
</body>
</html>